Conventionally, attempts have been made to remove self-interference in non-FDD (Frequency Division Duplex), full duplex systems (where simultaneous transmission and reception occurs in overlapping frequencies) by use of electronic means or a combination of passive and active electronics means. Recent work has focused on a solution for the case where the transmit and receive operations occur in the same frequency band. In traditional systems, either Time Domain Duplex (TDD) or FDD has been utilized. TDD allows for use of the same frequencies but at different times. FDD allows for use of two separate frequencies at the same time. In FDD, the energy in the receive band results from the nonlinearities of the power amplifier.
Other cancellation work has focused on “full-duplex” operation within the same frequency band at the same time. This is the type of scheme that has relied upon balun-type cancellers. The balun-based methods assume a certain group delay between the transmit and receive antennas. Balun-based or transformer-based coupler are employed to provide to the receiver canceller a sample of the interference that is 180° out-of-phase and free from additional group delay. In such conventional balun based systems, the exact delay needs to be controlled.
Thus there is a need to address the non-linearities and the exact group delay control issues seen in the conventional systems.